We tend to think of glass as a very stable material that doesn’t corrode, but that isn’t always true. Glass can and does corrode. The chemical that is most harmful to glass is something we consider fairly harmless, namely water. Water leaches out the alkali components (sodium and potassium) from the glass causing microscopic damage. This process generally takes time, so washing your glasses in water is safe. The composition of the glass is also a contributing factor. Some glasses have a much more stable composition than others.
The iridescence and discoloration seen on many archaeological glasses is a form of glass corrosion known as weathering.
During burial, moisture in the ground leaches out the alkali components from the glass, leaving behind distinct silica-rich layers alternating with layers of air. These layers are usually extremely thin, but numerous. They interfere with the direct transmission of light through the glass which causes the iridescence. The layers may be uniform and compact, or flaky, fragile, and discontinuous.
The discoloration of the glass is caused by the migration or alteration of coloring ions or other trace elements. The ions can be leached out of the glass or be taken up from the environment. For example, iron and manganese turn black, while contact with copper corrosion can cause green staining. Certain ions, most notably manganese and copper, may change color through oxidation.
The burial conditions and the composition of the glass both contribute to the extent and appearance of the weathering which can vary extensively even within a single piece of glass. Glass buried in dry environments will have little to no corrosion, while glass in moist burial environment will generally weather extensively. The acidity of the burial also influences the extent of corrosion. Glass is fairly resistant to acids and even highly acid (low pH) environments will do little damage to the glass unless fluoride or phosphate ions are present. Alkali (high pH) environments are much more damaging to glass because the silica network is attacked and broken down.
The thickness of the weathering can vary greatly depending on the chemical stability of the glass and the aggressiveness of the burial conditions. In extreme cases corrosion products may have completely replaced the original glass. Underneath the weathering the so-called glass core retains the original composition and color of the glass.
One may be tempted to remove the weathering to reveal the original color, and that was certainly done in the past. However, unlike corrosion on metals, glass corrodes from the outside inward and the weathering preserves the original surface. Any details of the surface such as tool or usage marks or even fine decoration will be lost if the weathering is removed. The glass underneath the weathering is usually unevenly preserved and may be pitted or appear etched. The original smooth surface of the statuette of Venus in the image to the left is perserved in the intact weathering, but the core glass exposed where the weathering has been lost is severely pitted.
Weathering is a type of corrosion found on archaeological glass, but historical, modern, and contemporary glasses are affected by another type of deterioration known as atmospheric corrosion, crizzling, weeping, or glass disease. I’ll save my discussion on that for another time.